Chlamydia trachomatis genital tract infections have a prevalence of 2-15% in adolescents/young adults in the USA and Europe in spite of public health efforts and effective antibiotic therapy. It is widely accepted that a vaccine is necessary reduce its prevalence. An early C.trachomatis trachoma (eye infection) vaccine delivered poor protection and exacerbated immunopathology. Critical to developing a safe Chlamydia vaccine is a better understanding of immunopathology. Currently there are no practicable biomarkers for Chlamydia immunopathology. In the C. muridarum mouse model others have clearly shown that Chlamydia-specific CD8 T cells are important mediators of immunopathology and infertility. We have recently shown that a subset of Chlamydia-specific CD8 T cell clones derived from mice that self-cleared C. muridarum genital tract infections produce the scar-ogenic cytokines IL-10, TNF?nd IL-13. These CD8 clones are not restricted by MHC class Ia molecules. Similar non-class Ia restricted CD8 T cell clones have been derived from the peripheral blood of humans with C. trachomatis genital tract infections. Using gene expression microarray analysis we have identified a molecular fingerprint for CD8IL-13 T cells. Based on the existing mouse and human Chlamydia literature it clear that the cellular immune response to Chlamydia genital tract infections includes non-class Ia restricted CD8 T cells. The C. muridarum mouse model has unequivocally shown CD8 T cells to be mediators of immunopathology and infertility. We hypothesize that the mechanism underlying CD8 immunopathology is a non-MHC class Ia restricted CD8 T subset secreting IL-10, TNF?and IL-13. To test that hypothesis and investigate the underlying immunobiology we propose the following specific aims: Specific aim #1- to investigate the role of CD8IL-13 T cells in immunopathology utilizing adoptive transfer. We have representative conventional Chlamydia-specific CD8 T cell clones, Chlamydia-specific CD8IL-13 T cell clones, and putative cell surface biomarkers for purifying CD8IL-13 T cells from immune splenocytes or bulk T cell populations. Specific aim #2- to investigate the activation pathway and other immunobiology specific to Chlamydia-specific CD8IL-13 T cells. Utilizing existing T cell clones and gene expression microarray analysis we will compare activated conventional CD8 with activated CD8IL-13 T cells to identify CD8IL-13 specific immunobiology; potentially identifying additional biomarkers and targets for therapeutic intervention. We will als map the epitope source proteins for the Chlamydia-specific CD8 T cell clone panel. Specific aim #3- perform a preliminary investigation of CD8IL-13 T cells in humans. Using putative biomarkers for the CD8IL-13 T cell subset identified in the mouse model we will isolate CD8 T cells from the peripheral blood of healthy individuals and determine whether a similar CD8IL-13 T cell subset exists in humans.